1. Field of the Invention
The present invention relates to a beam homogenizer for homogenizing a beam spot on an irradiation surface in a particular region. Moreover, the present invention relates to a laser irradiation apparatus to irradiate the irradiation surface with the beam spot.
2. Background Art
In recent years, a technique has been widely researched in which laser annealing is performed to crystallize a non-single crystal semiconductor film formed over an insulating substrate such as a glass substrate or to enhance the crystallinity of the non-single crystal semiconductor film. The non-single crystal semiconductor film is, for example, an amorphous semiconductor film (a-Si:H) or a crystalline semiconductor film (a semiconductor film having crystallinity such as a poly-crystalline semiconductor film and a microcrystal semiconductor film (μc-Si:H)). A silicon film is often employed as the semiconductor film.
In comparison with a quartz substrate which has been often used conventionally, the glass substrate is inexpensive and superior in workability, and therefore the glass substrate has an advantage that it can be processed easily into a large substrate. This is the reason why the research as above has been conducted. Moreover, a laser is often employed in the crystallization because the glass substrate has a low melting point. The laser can give high energy only to the non-single crystal semiconductor film without increasing the temperature of the substrate that much.
The crystalline silicon film is referred to as a poly-crystalline silicon film or a poly-crystalline semiconductor film because it has plenty of crystal grains. Since the crystalline silicon film formed by the laser annealing has high mobility, it is often used in a monolithic liquid crystal electro-optic device and the like. In such devices, the crystalline silicon film is used in a thin film transistor (TFT), and the TFT for driving a pixel and the TFT for a driver circuit are manufactured over a glass substrate.
It is preferable to perform the laser annealing in such a way that a pulsed laser beam emitted from an excimer laser or the like that has high output power is shaped into a square spot having a length of several cm on a side or a line having a length of 10 cm or more on the irradiation surface by an optical system and then the beam spot is scanned (the irradiation position of the laser beam is moved relative to the irradiation surface) because this method has high productivity and is superior industrially.
Particularly, when the laser beam has a linear shape, unlike a punctate beam spot that requires to be scanned from front to back and from side to side, the linear beam spot may be scanned only in a direction perpendicular to a long-side direction of the beam spot to perform the laser irradiation all over the irradiated substrate. Therefore, high productivity can be obtained. The linear beam spot is scanned in the direction perpendicular to the long-side direction because this is the most effective scanning direction. Because of this high productivity, the laser annealing is mainly employing the linear laser beam formed by shaping the pulsed excimer laser beam through an appropriate optical system.
FIGS. 1A and 1B show an example of an optical system for shaping a cross section of the laser beam into linear on the irradiation surface. This optical system not only shapes the cross section of the laser beam into linear but also homogenizes the energy of the laser beam on the irradiation surface simultaneously. Generally, the optical system for homogenizing the energy of the laser beam by using an optical element (a cylindrical lens, a doublet lens, or the like) is referred to as a beam homogenizer.
First, a side view of FIG. 1A is described. A laser beam emitted from a laser oscillator 101 is divided in a direction perpendicular to a traveling direction of a laser beam by cylindrical lens arrays 102a and 102b. This direction is hereinafter referred to as a vertical direction. In this constitution, the laser beam is divided into four beams. These divided beams are converged into one beam once by a cylindrical lens 104. Then, after the laser beam is reflected on a mirror 106, the laser beams are converged again into one laser beam on an irradiation surface 108 by a doublet cylindrical lens 107. A doublet cylindrical lens is a set of lenses consisting of two cylindrical lenses. This homogenizes the energy of the linear laser beam in a short-side direction and determines the length thereof in the short-side direction.
Next, a top view of FIG. 1B is described. The laser beam oscillated from the laser oscillator 101 is divided by a cylindrical lens array 103 in a direction that is perpendicular to the traveling direction of the laser beam and that is perpendicular to the vertical direction. The direction perpendicular to the vertical direction is hereinafter referred to as a horizontal direction. The laser beam is divided into seven beams in this constitution. After that, the divided beams are combined into one beam on the irradiation surface 108 by a cylindrical lens 105. This homogenizes the energy of the linear laser beam in a long-side direction and determines the length thereof in the long-side direction.
Each of the lenses is made of quartz in order to correspond to the excimer laser. In addition, the surfaces of the lenses are coated so that the laser beam emitted from the excimer laser transmits through the lenses very much. This makes transmittance of the excimer laser beam 99% or more per one lens.
The non-single crystal silicon film is irradiated with the linear laser beam formed by the above constitution as being overlapped in such a way that the linear beam is displaced gradually in the short-side direction of the linear laser beam. Accordingly, the laser annealing can be performed all over the surface of the non-single crystal silicon film so as to crystallize it or to enhance its crystallinity.
A poly-crystalline silicon film is obtained by shaping the pulsed excimer laser beam into linear through the optical system described above and irradiating, for example, the non-single crystal silicon film with the linear laser beam while scanning the linear laser beam.
In the obtained poly-crystalline silicon film, horizontal and vertical stripes are observed. When the poly-crystalline silicon film having such stripes is used to manufacture a display device with a driver and a pixel integrated (system-on-panel), the stripes directly appear on a screen because the semiconductor characteristic differs in each of the stripes. The stripes on the screen result mainly from the inhomogeneous crystallinity in the pixel portion. This problem can be reduced by improving the laser beam or the quality of the non-single crystal silicon film, which is the irradiation surface of the laser beam.
Particularly, the stripes appearing in parallel with the scanning direction of the linear laser beam mainly result from the design of the optical system for shaping the laser beam. Although various optical systems have been designed for this reason, it is difficult to design the optical system to form a desired laser beam because the optical elements constituting the optical system have their own limits in the arrangement depending on their characteristics.